feat(alerting): Add gotify provider (#605)

* feat(alerting): add gotify provider feature

* feat(alerting): update alert message

* feat(test): add tests for gotify provider

* feat(docs): add documentation for gotify provider

* feat(alerting): rename apptoken to token

* feat(docs): update docs for apptoken renaming to token

---------

Co-authored-by: Bugra Kocabay <kocabay.bugra@gmail.com>
Co-authored-by: TwiN <twin@linux.com>

alerting/provider/gotify/gotify.go

@@ -0,0 +1,105 @@
+package gotify
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+
+	"github.com/TwiN/gatus/v5/alerting/alert"
+	"github.com/TwiN/gatus/v5/client"
+	"github.com/TwiN/gatus/v5/core"
+)
+
+const DefaultPriority = 5
+
+// AlertProvider is the configuration necessary for sending an alert using Gotify
+type AlertProvider struct {
+	// ServerURL is the URL of the Gotify server
+	ServerURL string `yaml:"server-url"`
+
+	// Token is the token to use when sending a message to the Gotify server
+	Token string `yaml:"token"`
+
+	// Priority is the priority of the message
+	Priority int `yaml:"priority,omitempty"` // Defaults to DefaultPriority
+
+	// DefaultAlert is the default alert configuration to use for endpoints with an alert of the appropriate type
+	DefaultAlert *alert.Alert `yaml:"default-alert,omitempty"`
+
+	// Title is the title of the message that will be sent
+	Title string `yaml:"title,omitempty"`
+}
+
+// IsValid returns whether the provider's configuration is valid
+func (provider *AlertProvider) IsValid() bool {
+	if provider.Priority == 0 {
+		provider.Priority = DefaultPriority
+	}
+	return len(provider.ServerURL) > 0 && len(provider.Token) > 0
+}
+
+// Send an alert using the provider
+func (provider *AlertProvider) Send(endpoint *core.Endpoint, alert *alert.Alert, result *core.Result, resolved bool) error {
+	buffer := bytes.NewBuffer(provider.buildRequestBody(endpoint, alert, result, resolved))
+	request, err := http.NewRequest(http.MethodPost, provider.ServerURL+"/message?token="+provider.Token, buffer)
+	if err != nil {
+		return err
+	}
+	request.Header.Set("Content-Type", "application/json")
+	response, err := client.GetHTTPClient(nil).Do(request)
+	if err != nil {
+		return err
+	}
+	defer response.Body.Close()
+	if response.StatusCode > 399 {
+		body, _ := io.ReadAll(response.Body)
+		return fmt.Errorf("failed to send alert to Gotify: %s", string(body))
+	}
+	return nil
+}
+
+type Body struct {
+	Message  string `json:"message"`
+	Title    string `json:"title"`
+	Priority int    `json:"priority"`
+}
+
+// buildRequestBody builds the request body for the provider
+func (provider *AlertProvider) buildRequestBody(endpoint *core.Endpoint, alert *alert.Alert, result *core.Result, resolved bool) []byte {
+	var message, results string
+	if resolved {
+		message = fmt.Sprintf("An alert for `%s` has been resolved after passing successfully %d time(s) in a row", endpoint.DisplayName(), alert.SuccessThreshold)
+	} else {
+		message = fmt.Sprintf("An alert for `%s` has been triggered due to having failed %d time(s) in a row", endpoint.DisplayName(), alert.FailureThreshold)
+	}
+	for _, conditionResult := range result.ConditionResults {
+		var prefix string
+		if conditionResult.Success {
+			prefix = "✓"
+		} else {
+			prefix = "✕"
+		}
+		results += fmt.Sprintf("\n%s - %s", prefix, conditionResult.Condition)
+	}
+	if len(alert.GetDescription()) > 0 {
+		message += " with the following description: " + alert.GetDescription()
+	}
+	message += results
+	title := "Gatus: " + endpoint.DisplayName()
+	if provider.Title != "" {
+		title = provider.Title
+	}
+	body, _ := json.Marshal(Body{
+		Message:  message,
+		Title:    title,
+		Priority: provider.Priority,
+	})
+	return body
+}
+
+// GetDefaultAlert returns the provider's default alert configuration
+func (provider AlertProvider) GetDefaultAlert() *alert.Alert {
+	return provider.DefaultAlert
+}